Plasma generating electrode, plasma generation device, and exhaust gas purifying device

ABSTRACT

A plasma generating electrode of the invention present includes at least a pair of electrodes  5,  at least one electrode  5   a  of the pair of electrodes  5  including a plate-like ceramic body  2  as a dielectric and a conductive film  3  disposed inside the ceramic plate  2  and having a plurality of through-holes  4  formed through the conductive film  3  in its thickness direction, the through-holes having a cross-sectional shape including an arc shape along a plane perpendicular to the thickness direction. The plasma generating electrode can generate uniform and stable plasma at low power consumption.

TECHNICAL FIELD

The present invention relates to a plasma generating electrode, a plasmageneration device, and an exhaust gas purifying device. Moreparticularly, the invention relates to a plasma generating electrode anda plasma generation device capable of generating uniform and stableplasma at low power consumption. The invention also relates to anexhaust gas purifying device capable of purifying exhaust gas well.

BACKGROUND ART

It is known that silent discharge occurs when disposing a dielectricbetween two electrodes and applying a high alternating current voltageor a periodic pulsed voltage between the electrodes and that, in theresulting plasma field, active species, radicals, and ions are producedto promote reaction and decomposition of gas. This phenomenon may beutilized to remove toxic components contained in engine exhaust gas orincinerator exhaust gas.

For example, a plasma exhaust gas treatment system has been disclosed inwhich NO_(x), carbon fine particles, HC, and CO contained in engineexhaust gas or incinerator exhaust gas is oxidized by causing the engineexhaust gas or incinerator exhaust gas to pass through plasma (e.g.JP-A-2001-164925).

DISCLOSURE OF THE INVENTION

However, since such a plasma generating electrode used to generateplasma causes a local point discharge to occur between a pair ofelectrodes disposed opposite to each other, there arises a problem thatuniform plasma cannot be generated over the entire electrode.

The present invention has been achieved in view of the above-describedproblem, and provides a plasma generating electrode and a plasmageneration device capable of generating uniform and stable plasma at lowpower consumption. The invention also provides an exhaust gas purifyingdevice which includes the above plasma generation device and a catalystand can reliably purify exhaust gas.

In order to achieve the above objects, the invention provides thefollowing plasma generating electrode, plasma generation device, andexhaust gas purifying device.

[1] A plasma generating electrode comprising at least a pair ofelectrodes disposed opposite to each other and generating plasma uponapplication of voltage between the pair of electrodes, at least one ofthe pair of electrodes including a plate-like ceramic body as adielectric and a conductive film disposed inside the ceramic plate andhaving a plurality of through-holes formed through the conductive filmin its thickness direction, the through-holes having a cross-sectionalshape including an arc shape along a plane perpendicular to thethickness direction (hereinafter may be called “first invention”).

[2] The plasma generating electrode according to [1], wherein thethrough-holes have a circular cross-sectional shape along a planeperpendicular to the thickness direction.

[3] The plasma generating electrode according to [1] or [2], wherein thethrough-holes are regularly arranged in the conductive film.

[4] The plasma generating electrode according to any of [1] to [3],wherein the conductive film is disposed inside the ceramic formed bodyby screen printing, calender rolling, spraying, chemical vapordeposition, or physical vapor deposition.

[5] The plasma generating electrode according to any of [1] to [4],wherein the through-holes have a diameter of 1 to 10 mm.

[6] The plasma generating electrode according to any of [1] to [5],wherein a center-to-center distance between the adjacent through-holesis 1.5 to 20 mm.

[7] The plasma generating electrode according to any of [1] to [6],wherein the conductive film includes at least one metal selected fromthe group consisting of tungsten, molybdenum, manganese, chromium,titanium, zirconium, nickel, iron, silver, copper, platinum, andpalladium as a major component.

[8] A plasma generation device comprising the plasma generatingelectrode according to any of [1] to [7] (hereinafter may be called“second invention”).

[9] An exhaust gas purifying device comprising the plasma generationdevice according to [8] and a catalyst, the plasma generation device andthe catalyst being disposed inside an exhaust system of an internalcombustion engine (hereinafter may be called “third invention”).

BRIEF DESCRIPTION OF THE DRAWINGS

[FIG. 1] FIG. 1 is a perspective view schematically showing oneembodiment of a plasma generating electrode according to the presentinvention (first invention).

[FIG. 2] FIG. 2 is a plan view schematically showing an example of aceramic formed body and a conductive film constituting an electrode inone embodiment of a plasma generating electrode according to the presentinvention (first invention).

[FIG. 3] FIG. 3 is a perspective view schematically showing anotherembodiment of a plasma generating electrode according to the invention(first invention).

[FIG. 4] FIG. 4 is a plan view schematically showing another example ofa ceramic formed body and a conductive film constituting the electrodein one embodiment of the plasma generating electrode according to thepresent invention (first invention).

[FIG. 5( a)] FIG. 5( a) is a cross-sectional view showing one embodimentof a plasma generation device according to the present invention (secondinvention) along a plane including the treatment target fluid flowdirection.

[FIG. 5( b)] FIG. 5( b) is a cross-sectional diagram along the line A-Ashown in FIG. 5( a).

[FIG. 6] FIG. 6 is an explanatory view schematically showing oneembodiment of an exhaust gas purifying device according to the presentinvention (third invention).

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of a plasma generating electrode, a plasma generationdevice, and an exhaust gas purifying device according to the presentinvention are described below in detail with reference to the drawings.

FIG. 1 is a perspective view schematically showing one embodiment of aplasma generating electrode according to the present invention (firstinvention), and FIG. 2 is a plan view schematically showing a ceramicbody and a conductive film constituting one electrode of a plasmagenerating electrode. As shown in FIGS. 1 and 2, a plasma generatingelectrode 1 according to the present embodiment includes at least a pairof electrodes 5 disposed opposite to each other and generates plasmaupon application of voltage between the electrodes 5, at least oneelectrode 5 a of the pair of electrodes 5 including a plate-like ceramicbody 2 as a dielectric and a conductive film 3 disposed inside theceramic plate 2 and having a plurality of through-holes 4 formed throughthe conductive film 3 in its thickness direction, the through-holeshaving a cross-sectional shape including an arc shape along a planeperpendicular to the thickness direction (hereinafter may be called“cross-sectional shape of the through-hole”). In the present embodiment,the configuration of the other electrode is not particularly limited. Asshown in FIG. 1, a conventionally known metal electrode may be used. Asshown in FIG. 3, it is preferable that the other electrode 5 b of theplasma generating electrode 1 include a conductive film having aplurality of through-holes formed through the conductive film in itsthickness direction and having a cross-sectional shape including an arcshape along a plane perpendicular to the thickness direction. In thiscase, it is preferable that connection sections for respectively causingcurrent to flow through the electrode 5 a and the electrode 5 b beformed in opposite directions.

In the plasma generating electrode 1 shown in FIG. 1, two electrodes 5are disposed opposite to each other. However, the number of electrodes 5is not limited to two. For example, three or more electrodes may bedisposed in parallel so that adjacent electrodes respectively form apair of electrodes (not shown).

FIGS. 1 and 2 illustrate the through-hole 4 having a circularcross-sectional shape along a plane perpendicular to the thicknessdirection. However, the cross-sectional shape of the through-hole 4 isnot limited to circular, but may be an ellipse, a polygon having thevertices which are rounded off, or the like.

The plasma generating electrode 1 according to the present embodiment isa barrier discharge type plasma generating electrode 1 including theceramic plate 2 as a dielectric and the conductive film 3 disposedinside the ceramic plate 2. The plasma generating electrode 1 maysuitably be used for an exhaust gas treatment device or an exhaust gaspurifying device which treats a treatment target fluid such as exhaustgas by causing the treatment target fluid to pass through plasmagenerated between a pair of electrodes 5, or an ozonizer which producesozone by reacting oxygen contained in air, for example.

Conventionally, the barrier discharge type electrode has been consideredto generate relatively uniform plasma due to occurrence of dischargeover the entire surface of the dielectric. In practice, discharge doesnot occur in such a manner that the potential is equal over the entiresurface of the dielectric. When the conductor (conductive film) is inthe shape of a sheet, local point discharge occurs at unspecified pointsof the dielectric so that uniform plasma cannot be generated. When theconductor (conductive film) is in the shape of a mesh, discharge isconcentrated at positions corresponding to the intersection points ofthe mesh so that uniform plasma cannot be generated. In the presentembodiment, since the through-holes 4 having a cross-sectional shapeincluding an arc shape along a plane perpendicular to the thicknessdirection of the conductive film 3 are formed in the conductive film 3constituting the plasma generating electrode 1, the boundary between thethrough-hole 4 and the conductive film acts as a discharge startingpoint so that discharge can be uniformly caused to occur at the outerperiphery of the through-hole 4. Moreover, since the through-holes 4 areformed in the entire conductive film, stable and uniform plasma can begenerated between a pair of electrodes 5. If the cross-sectional shapeof the through-hole 4 along a plane perpendicular to the thicknessdirection is polygonal or the like, a discharge is concentrated atpositions corresponding to the vertices of the polygon or the like, sothat uniform plasma cannot be generated.

The size of the through-hole 4 is not particularly limited. For example,it is preferable that the diameter of the through-hole 4 be 1 to 10 mm.This configuration allows electric field concentration at the outerperiphery of the through-hole 4 to be appropriate for discharge, so thatdischarge occurs well even if the voltage applied between the pair ofelectrodes 5 is not so high. If the diameter of the through-hole 4 isless than 1 mm, discharge occurring at the outer periphery of thethrough-hole 4 becomes similar to the above-described local pointdischarge, so that nonuniform plasma may be generated. If the diameterof the through-hole 4 is more than 10 mm, since discharge hardly occursinside the through-hole 4, the density of plasma generated between thepair of electrodes 5 may be decreased.

In the present embodiment, it is preferable that the center-to-centerdistance between the adjacent through-holes 4 be appropriatelydetermined according to the diameters of the through-holes 4 so thatuniform plasma can be generated at high density. For example, it ispreferable that the center-to-center distance between the adjacentthrough-holes 4 be 1.5 to 20 mm, although the center-to-center distanceis not limited thereto.

It is preferable that the through-hole 4 be formed so that the length ofthe outer periphery of the through-hole 4 per unit area is increased.This configuration enables length of the region in which a nonuniformelectric field occurs, that is, the outer periphery acting as a plasmageneration point, to be increased per unit area, so that a large amountof discharge per unit area is caused, whereby plasma can be generated athigh density. A specific length of the outer periphery of thethrough-hole 4 per unit area (mm/mm²) may appropriately be determineddepending on the intensity of plasma to be generated or the like. In thecase of treating automotive exhaust gas, the length of the outerperiphery of the through-hole 4 per unit area is preferably 0.05 to 1.7mm/mm². If the length of the outer periphery of the through-hole 4 perunit area is less than 0.05 mm/mm², local discharge may occur so that itmay become difficult to obtain a stable discharge space. If the lengthof the outer periphery of the through-hole 4 per unit area is more than1.7 mm/mm², the resistance of the conductive film may be increased,whereby discharge efficiency may be decreased.

In the present embodiment, it is preferable that the area of theconductive film 3 per unit area be 0.1 to 0.98 mm²/mm². If the area ofthe conductive film 3 per unit area is less than 0.1 mm²/mm², it maybecome difficult to cause discharge to occur in an amount necessary forpurifying exhaust gas due to too small electrostatic capacitance of thedielectric electrode. If the area of the conductive film 3 per unit areais more than 0.98 mm²/mm², it may be difficult to obtain a uniformdischarge effect due to the through-holes, so that local discharge mayeasily occur.

In more detail, for determining the length of the outer periphery of thethrough-hole 4 and the area of the conductive film 3 per unit area, inthe case of treating soot contained in automotive exhaust gas, it ispreferable that the length of the outer periphery of the through-hole 4per unit area be 1.0 mm/mm² or less and the area of the conductive film3 per unit area be 0.2 mm²/mm² or more. In the case of treating nitrogenoxide (NO_(x)) contained in exhaust gas, it is preferable that thelength of the outer periphery of the through-hole 4 per unit area be 0.2mm/mm² or more and the area of the conductive film 3 per unit area be0.9 mm²/mm² or less.

It is preferable that the conductive film 3 has a thicknesscorresponding to 0.1 to 10% of the thickness of the ceramic plate 2.This configuration allows uniform discharge to occur over the surface ofthe ceramic plate 2 as a dielectric. Specifically, it is preferable thatthe thickness of the conductive film 3 is about 5 to 50 μm in order toreduce the size of the plasma generating electrode 1 and reduce theresistance of a treatment target fluid, such as exhaust gas, which iscaused to pass through the space between the pair of electrodes 5. Ifthe thickness of the conductive film 3 is less than 5 μm, reliabilitymay be decreased in the case of forming the conductive film 3 byprinting or the like. Moreover, since the resistance of the resultingconductive film 3 may be increased, the plasma generation efficiency maybe decreased. If the thickness of the conductive film 3 is more than 50μm, the resistance of the conductive film 3 is reduced. However, sincethe conductive film 3 having such a thickness affects the uniformity ofthe surface of the ceramic plate 2, it may be necessary to process thesurface of the ceramic plate 2 so that the surface becomes flat.

In the present embodiment, it is preferable that the conductive film 3constituting the electrode 5 a be disposed inside the ceramic plate 2 sothat the conductive film 3 is positioned approximately at an equaldistance from both the surfaces of the ceramic plate 2. Thisconfiguration enables plasma to be generated at an equal intensitybetween adjacent electrodes, even in the case of generating plasma in astate in which a plurality of electrodes are consecutively disposedopposite to one another. When the conductive film 3 is disposed so thatthe distances from both the surfaces of the ceramic plate 2 differ, theelectrostatic capacitance differs between the surfaces of the electrode5 a, so that the discharge characteristics may differ between thesurfaces.

The conductive film 3 used in the present embodiment preferably includesa metal exhibiting excellent conductivity as the major component. Assuitable examples of the major component of the conductive film 3, atleast one metal selected from the group consisting of tungsten,molybdenum, manganese, chromium, titanium, zirconium, nickel, iron,silver, copper, platinum, and palladium can be given. In the presentembodiment, the term “major component” refers to a component accountingfor 60 mass % or more of the components. When the conductive film 3includes two or more metals selected from the above-mentioned group asthe major component, the total amount of the metals accounts for 60 mass% or more of the components.

As a method of disposing the conductive film 3 inside the ceramic plate2, a method of embedding the conductive film 3, such as a metal plate ormetal foil, in a press-formed body obtained by powder press forming canbe given, for example. In more detail, in the case of forming apress-formed body (ceramic body) by powder press forming, a metal plateor metal foil containing the above-mentioned metal as the majorcomponent is embedded so that the metal plate or metal foil is disposedat an equal distance (distance in the thickness direction) from thesurfaces of the press-formed body. Since the embedded metal foil or thelike may be deformed or cut due to firing shrinkage of the ceramics, itis preferable to sinter the press-formed body so that mass transfer isprevented in the horizontal (planar) direction. In this case, thepress-formed body may be sintered while applying pressure to thepress-formed body in the thickness direction.

The conductive film 3 may be applied to the ceramic plate 2. As suitableexamples of the method of applying the conductive film 3, screenprinting, calender rolling, chemical vapor deposition, and physicalvapor deposition can be given. According to these methods, a conductivefilm 3 exhibiting excellent surface flatness and smoothness afterapplication and having a small thickness can be easily formed. Among theabove application methods, chemical vapor deposition and physical vapordeposition may increase a cost. However, these methods enable a thinnerconductive film to be easily disposed and through-holes having a smallerdiameter and a smaller center-to-center distance to be easily formed.

In the case of applying the conductive film 3 to the ceramic plate 2, apowder of a metal mentioned above as the major component of theconductive film 3, an organic binder, and a solvent such as terpineolmay be mixed together to form a conductive paste, and the conductivepaste may be applied to the ceramic plate 2 by using the above-describedmethod. An additive may optionally be added to the conductive paste inorder to improve adhesion to the ceramic plate 2 and improvesinterability.

The adhesion between the conductive film 3 and the ceramic plate 2 canbe improved by adding the same component as that of the ceramic plate 2to the metal component of the conductive film 3. A glass component maybe added to the ceramic component added to the metal component. Theaddition of the glass component improves the sinterability of theconductive film 3 so that the density of the conductive film 3 isimproved in addition to adhesion. The total amount of the component ofthe ceramic plate 2 and/or the glass component other than the metalcomponent is preferably 30 mass % or less. If the total amount exceeds30 mass %, the function as the conductive film 3 may not obtained due todecrease in resistance.

The ceramic plate 2 in the present embodiment has the function as adielectric as described above. By using the conductive film 3 in a statein which the conductive film 3 is held inside the ceramic plate 2, localdischarge such as a spark can be reduced and small discharge can becaused to occur at a plurality of locations in comparison with the caseof causing discharge to occur by using the conductive film 3 alone. Suchsmall discharge can reduce power consumption, since the amount ofcurrent caused to flow is small in comparison with discharge such as aspark. Moreover, current which flows between the pair of electrodes 5 islimited due to the presence of the dielectric, so that nonthermal plasmaconsuming only a small amount of energy without increase in temperaturecan be generated.

The aforementioned ceramic body 2 preferably includes a material havinga high dielectric constant as the major component. As the material forthe ceramic plate 2, aluminum oxide, zirconium oxide, silicon oxide,titanium-barium type oxide, magnesium-calcium-titanium type oxide,barium-titanium-zinc type oxide, silicon nitride, aluminum nitride, orthe like may be suitably used. The plasma generating electrode 1 can beoperated at high temperature by using a material exhibiting excellentthermal shock resistance as the major component of the ceramic plate 2.

The thickness of the ceramic plate 2 is preferably 0.1 to 3 mm althoughthe thickness is not limited thereto. If the thickness of the ceramicplate 2 is less than 0.1 mm, it may be difficult to ensure the electricinsulating properties of the electrode 5. If the thickness of theceramic plate 2 is more than 3 mm, reduction in size of an exhaust gaspurifying system may be hindered. Moreover, the applied voltage must beincreased due to increase in the electrode-to-electrode distance,whereby the efficiency may be decreased.

As the ceramic plate 2 used in the present embodiment, a ceramic greensheet used for a ceramic substrate may suitably be used. The ceramicgreen sheet may be obtained by forming slurry or paste for a green sheetto have a predetermined thickness by using a conventionally known methodsuch as a doctor blade method, a calender method, a printing method, ora reverse roll coating method. The resulting ceramic green sheet may besubjected to cutting, shaving, punching, or formation of a communicatinghole, or may be used as an integral laminate in which the green sheetsare layered and bonded by thermocompression bonding or the like.

As the above slurry or paste for a green sheet, a mixture prepared bymixing an appropriate binder, sintering agent, plasticizer, dispersant,organic solvent, and the like into a predetermined ceramic powder may besuitably used. As suitable examples of the ceramic powder, alumina,mullite, ceramic glass, zirconia, cordierite, silicon nitride, aluminumnitride, glass, and the like can be given. As suitable examples of thesintering agent, silicon oxide, magnesium oxide, calcium oxide, titaniumoxide, zirconium oxide, and the like can be given. The sintering agentis preferably added in an amount of 3 to 10 parts by mass with respectto 100 parts by mass of the ceramic powder. As the plasticizer,dispersant, and organic solvent, those used for a known method maysuitably be used.

As the ceramic plate 2 used in the present embodiment, a ceramic sheetformed by extrusion may also be suitably used. For example, a plate-likeceramic formed body obtained by extruding by using a predetermined die amixture prepared by mixing the above-mentioned ceramic powder with aforming agent such as methyl cellulose, a surfactant, and the like maybe used.

In the present embodiment, the porosity of the ceramic formed body 2 ispreferably 0.1 to 35%, and more preferably 0.1 to 10%. Thisconfiguration allows plasma to be efficiently generated between theelectrode 5 a including the ceramic plate 2 and the other electrode 5 bdisposed opposite to the electrode 5 a, so that energy consumption canbe reduced.

It is preferable that the pair of electrodes 5 be disposed at such adistance that plasma can be effectively generated therebetween. Theelectrodes 5 are preferably disposed at a distance of 0.1 to 5 mmalthough the distance may differ depending on the voltage applied to theelectrodes or the like.

In the electrode 5 a shown in FIG. 2, the through-holes 4 are formed inthe conductive film 3 so that the straight lines connecting the centersof the adjacent through-holes 4 form an equilateral triangle. However,the through-holes 4 may be formed so that the straight lines connectingthe centers of the adjacent through-holes 4 form a square as shown inFIG. 4.

A method of manufacturing a plasma generating electrode of the presentembodiment is described below in detail.

First, a ceramic green sheet used for the ceramic plate is formed. Forexample, a sintering agent, a binder such as a butyral resin or acellulose resin, a plasticizer such as DOP or DBP, an organic solventsuch as toluene or butadiene, and the like are added to at least onematerial selected from the group consisting of alumina, mullite, ceramicglass, zirconia, cordierite, silicon nitride, aluminum nitride, andglass. The components are sufficiently mixed by using an alumina pot andalumina ball to prepare slurry for a green sheet. The slurry for a greensheet may be prepared by mixing the materials by ball milling using amono ball.

The resulting slurry for a green sheet is stirred under reduced pressurefor degassing, and adjusted to have a predetermined viscosity. Theresulting slurry for a green sheet is formed in the shape of a tape byusing a tape forming method such as a doctor blade method to form anunfired ceramic body.

Meanwhile, a conductive paste for forming a conductive film disposed onone surface of the unfired ceramic body is provided. The conductivepaste may be formed by adding a binder and a solvent such as terpineolto silver powder and sufficiently kneading the mixture by using atriroll mill, for example.

The resulting conductive paste is printed on the surface of the unfiredceramic body by screen printing or the like to form a conductive filmhaving a predetermined shape. At that time, the conductive paste isprinted so that through-holes having a circular cross-sectional shapeare formed in the conductive film. In order to externally supplyelectricity to the conductive film after holding the conductive filminside the ceramic plate, the conductive paste is printed so that theconductive film extends to the outer periphery of the unfired ceramicbody to secure an electricity supply section from the outside.

Another unfired ceramic formed-body is layered on the unfired ceramicbody on which the conductive film is printed so that the printedconductive film is covered. It is preferable to layer the unfiredceramic formed bodies at a temperature of 100° C. while applying apressure of 10 MPa.

Next, the unfired ceramic bodies layered with the conductive filminterposed therebetween are fired to form an electrode including aplate-like ceramic body as a dielectric and a conductive film disposedinside the ceramic plate and having a plurality of through-holes formedthrough the conductive film in its thickness direction and having across-sectional shape including an arc shape along a plane perpendicularto the thickness direction.

A counter electrode is disposed opposite to the resulting electrode toform a plasma generating electrode of the present embodiment. As thecounter electrode, an electrode obtained by using the above-describedmanufacturing method or an electrode having a conventionally knownconfiguration may be used.

One embodiment of a plasma generation device of the present invention(second invention) is described below. As shown in FIGS. 5( a) and 5(b),a plasma generation device 10 of the present embodiment includes theplasma generating electrode 1 of the first invention. In more detail,the plasma generation device 10 includes the plasma generating electrode1 and a casing 11 which accommodates the plasma generating electrode 1in a state in which a treatment target fluid such as exhaust gas canpass through the space between the pair of electrodes 5 constituting theplasma generating electrode 1. The casing includes an inlet port 12through which the treatment target fluid flows in, and an outlet port 13through which the treatment target fluid which has passed through thespace between the electrodes 5 and has been treated (treated fluid) isdischarged.

Since the plasma generation device 10 of the embodiment includes theplasma generating electrode 1 of the first invention, the plasmageneration device 10 can generate uniform and stable plasma at low powerconsumption.

As shown in FIGS. 5( a) and 5(b), it is preferable in the plasmageneration device 10 according to the embodiment that the plasmagenerating electrodes 1, each having a pair of electrodes 5, be disposedin layers inside the casing 11. FIGS. 5( a) and 5(b) illustrate thestate in which five plasma generating electrodes 1, each having a pairof electrodes 5, are layered for convenience of illustration. However,the number of plasma generating electrodes 1 to be layered is notlimited to thereto. A spacer 14 is disposed between the pair ofelectrodes 5 forming the plasma generating electrode 1 and between eachof the plasma generating electrodes 1 in order to form a predeterminedopening.

The plasma generation device 10 configured as described above may beinstalled in an automotive exhaust system, for example. In this case,exhaust gas discharged from an engine or the like is caused to passthrough plasma generated between the pair of electrodes 5 so that toxicsubstances such as soot and nitrogen oxide contained in the exhaust gasare reacted and discharged to the outside as a nonhazardous gas.

It is preferable to layer the plasma generating electrodes 1 so thatplasma can be generated between the layered plasma generating electrodes1. Specifically, it is preferable to configure the plasma generationdevice 10 so that discharge occurs between the electrode 5 a of theelectrode 5 constituting the plasma generating electrode 1 a and theelectrode 5 b disposed opposite to the electrode 5 a and that dischargealso occurs between the electrode 5 a constituting the electrode 5 ofthe plasma generating electrode 1 a and the electrode 5 b constitutingthe adjacent plasma generating electrode 1 b, such that plasma can begenerated between the layered plasma generating electrodes 1.

The plasma generation device of the present embodiment may include apower source for applying voltage to the plasma generating electrode(not shown). As the power source, a conventionally known power sourcemay be used insofar as the power supply can supply current which caneffectively generate plasma. For example, a pulse power source using athyristor, a pulse power source using a transistor other than athyristor, a general AC power source, or the like may suitably be used.

The plasma generation device of the present embodiment may be configuredso that current is supplied from an external power source instead ofproviding a power source inside the plasma generation device.

Current supplied to the plasma generating electrode used in the preesntembodiment may appropriately be selected depending on the intensity ofplasma to be generated. For example, in the case of installing theplasma generation device in an automotive exhaust system, it ispreferable that current supplied to the plasma generating electrode be adirect current at a voltage of 1 kV or more, a pulsed current having apeak voltage of 1 kV or more and a pulse rate per second of 100 or more(100 Hz or more), an alternating current having a peak voltage of 1 kVor more and a pulse rate per second of 100 Hz or more, or a currentgenerated by superimposing two of these currents. This enables efficientgeneration of plasma.

One embodiment of an exhaust gas purifying device of the presentinvention (third invention) is described below in detail. FIG. 6 is anexplanatory view schematically showing an exhaust gas purifying deviceaccording to the embodiment. As shown in FIG. 6, an exhaust gaspurifying device 41 of the present embodiment includes the plasmageneration device 10 according to the above-described embodiment of thesecond invention and a catalyst 44, the plasma generation device 10 andthe catalyst 44 being provided inside an exhaust system of an internalcombustion engine. The plasma generation device 10 is provided on theexhaust gas generation side (upstream) of the exhaust system, and thecatalyst 44 is provided on the exhaust side (downstream). The plasmageneration device 10 and the catalyst 44 are connected through a pipe42.

The exhaust gas purifying device 41 of the present embodiment is adevice which purifies NO_(x) in exhaust gas in an oxygen-excessatmosphere, for example. That is, NO_(x) is reformed by plasma generatedby the plasma generation device so that NO_(x) is easily purified by thedownstream catalyst 44, or a hydrocarbon (HC) or the like in exhaust gasis converted so that HC easily reacts with NO_(x), to purify NO_(x) bythe catalyst 44.

The plasma generation device 10 used in the exhaust gas purifying device41 of the present embodiment converts NO_(x) in exhaust gas generated bycombustion in an oxygen-excess atmosphere as in a lean burn or gasolinedirect injection engine, a diesel engine, or the like into NO₂. Theplasma generation device 10 generates active species from HC or the likecontained in exhaust gas. As the plasma generation device 10, a plasmageneration device configured in the same manner as the plasma generationdevice 10 shown in FIG. 5( a) may suitably be used.

The catalyst 44 is provided downstream of the plasma generation device10 in the exhaust system as a catalyst unit 45 provided with a catalyticmember including a substrate having pores through which exhaust gascirculates formed therein. The catalytic member includes the substrateand a catalyst layer formed to cover the inner walls surrounding thepores of the substrate.

The catalyst layer is generally formed by inpregnating the substratewith a catalyst in the form of slurry (catalyst slurry) as describedlater. Therefore, the catalyst layer may be called a “washcoat (layer)”.

The shape of the substrate is not particularly limited insofar as thesubstrate has an exhaust gas circulation space. The present embodimentuses a honeycomb-shaped substrate in which pores are formed.

It is preferable that the substrate be formed of a material exhibitingheat resistance. As examples of such a material, a porous material(ceramic) such as cordierite, mullite, silicon carbide (SiC), andsilicon nitride (Si₃N₄), a metal (e.g. stainless steel) and the like canbe given.

The catalyst layer mainly includes a porous carrier and one or acombination of two or more elements selected from Pt, Pd, Rh, Au, Ag,Cu, Fe, Ni, Ir, and G a supported on the surface of the porous carrier.A plurality of pores communicating with the pores in the substrate areformed in the catalyst layer.

The porous substrate may appropriately be formed of alumina, zeolite,silica, titania, zirconia, silica-alumina, ceria, or the like. As thecatalyst 44, a catalyst which promotes decomposition of NO_(x) is used.

The invention is described below in detail by way of examples. However,the invention should not be construed as being limited to the followingexamples.

EXAMPLE 1

A plasma generation device including the plasma generating electrode 1as shown in FIG. 1 was manufactured. The plasma generating electrode wasmanufactured by disposing two electrodes opposite to each other at adistance of 1 mm, each of the electrodes including a plate-like ceramicbody as a dielectric formed of an alumina tape, and a conductive filmdisposed inside the ceramic plate and having through-holes formedthrough the conductive film in its thickness direction and having acircular cross-sectional shape along a plane perpendicular to thethickness direction. One of the pair of electrodes of the plasmagenerating electrode was used as a voltage application side, and theother was used as a grounding side.

The ceramic plate had a length of 50 mm, a width of 90 mm, and athickness of 1 mm. The conductive film had a length of 40 mm, a width of80 mm, and a thickness of 20 μm. The through-holes had a diameter of 3mm and were equally formed so that the center-to-center distance was 5mm. The conductive film was formed by printing a metal paste containing95 mass % of tungsten on the surface of the ceramic plate and firing themetal paste together with the ceramic plate.

Five plasma generating electrodes thus obtained were layered so that thevoltage application side and the grounding side of the pair ofelectrodes were alternately disposed to obtain a plasma generationdevice. The plasma generating electrodes were layered so that thedistance between the plasma generating electrodes was 1 mm.

A pulse power source using a thyristor was connected with the voltageapplication side electrode of the plasma generating electrode, and thegrounding side electrode was grounded.

The plasma generation device of the present example (Example 1) waselectrified at a voltage of 5 kV and a frequency of 500 Hz. As a result,uniform and stable plasma could be generated.

A mixed gas prepared by mixing NO gas into gas in which the ratio of N₂and O₂ was adjusted to the same ratio as in air was caused to passthrough plasma generated by the plasma generation device of the presentexample to evaluate the conversion efficiency of NO contained in themixed gas into NO₂.

Specifically, NO was added to a gas stream (50 NL/min) at roomtemperature to prepare a mixed gas having an NO concentration of 200ppm. The mixed gas was caused to pass through plasma generated by usingthe plasma generation device of the present example. The plasma wasgenerated at a voltage of 6 kV and a frequency of 500 Hz.

The NO concentration of the mixed gas after being passed through plasmawas reduced to 85 ppm. The mixed gas having an NO concentration of 200ppm was caused to pass through plasma generated at a voltage of 7 kV(power consumption: 25 W). As a result, the NO concentration was reducedto 2 ppm indicating that almost the total amount of NO was convertedinto NO₂. It is difficult to convert NO contained in exhaust gas into N₂and O₂ by using an exhaust gas treatment catalyst at low temperaturearound room temperature. However, the purification of NO is facilitatedby converting NO into NO₂ through plasma, so that clean gas can beeasily obtained.

COMPARATIVE EXAMPLE 1

A plasma generation device was manufactured in the same manner as theplasma generation device of Example 1 except that the through-holes werenot formed.

The plasma generation device was electrified at a voltage of 7 kV and afrequency of 500 Hz by using a pulse power source using a thyristor, anda mixed gas having an NO concentration of 200 ppm was caused to passthrough plasma in the same manner as in Example 1. As a result, the NOconcentration was reduced to only 50 ppm. In Comparative Example 1,discharge occurred at arbitrary locations on the surface of theelectrode when plasma was generated. Therefore, since discharge did notoccur in the entire space, nonuniform plasma was generated. Thoughdischarge occurred in the entire space by injecting high energy byincreasing the voltage to 8 kV, high voltage and high power wasnecessary in comparison with the electrode having the through-holes.

EXAMPLE 2

A plasma generation device was manufactured in the same manner as theplasma generation device of Example 1 except for disposing circularthrough-holes having a diameter of 5 mm at a center-to-center distanceof 6 mm.

When causing a mixed gas similar to that described above to pass throughthe plasma generation device of the present example (Example 2), the NOconcentration was reduced to 3 ppm at a power consumption of 18 W. Thisplasma generation device could convert NO at low power consumption incomparison with the plasma generation device of Example 1 so that highenergy efficiency was obtained. This indicates that the diameter and thecenter-to-center distance of the through-holes affect power required togenerate plasma.

EXAMPLE 3

A plasma generation device was manufactured in the same manner as theplasma generation device of Example 1 except for using a stainless steelelectrode as one of the pair of electrodes constituting the plasmagenerating electrode.

The same mixed gas was caused to pass through the plasma generationdevice of the present example (Example 3). Electrification was performedat a voltage of 6 kV and a frequency of 500 Hz, and a mixed gas havingan NO concentration of 200 ppm was caused to pass through plasmagenerated. As a result, the NO concentration was reduced to 5 ppm. Atthis time, the amount of power supplied to the plasma generation devicewas 40 W, so that power consumption was higher than that of Example 1.However, NO could be converted with high efficiency.

EXAMPLE 4

An AC power source was connected with a plasma generation devicemanufactured in the same manner as the plasma generation device ofExample 1, and an NO conversion efficiency test was conducted. In thecase of generating plasma by supplying electricity at a voltage of ±7 kVand a frequency of 500 Hz (sine wave), the NO concentration was reducedto 100 ppm. By supplying electricity at a voltage of ±7 kV and afrequency of 1 kHz (sine wave), the NO concentration was reduced to 10ppm. Thus, plasma could effectively be generated also by using the ACpower source.

EXAMPLE 5

A plasma generation device was manufactured in the same manner as theplasma generation device of Example 1 except for changing theelectrode-to-electrode distance to 0.5 mm.

In order to evaluate the exhaust gas carbon particulate purificationperformance, soot was caused to flow through the plasma generationdevice at a flow rate of 5 g/hr, and the amount of carbon particulatetrapped at the discharge port of the plasma generation device wasevaluated.

Plasma was generated by supplying electricity at a voltage of 5 kV and afrequency 250 Hz by using a pulse power source using an SI thyristor,and the purification rate calculated from the amount of carbonparticulate trapped was 60%. In the case of supplying electricity at avoltage of 5 kV and a frequency of 500 Hz, the carbon particulatepurification rate was increased to 90%. Therefore, it was confirmed thatthe plasma generation device of the present example (Example 5) iseffective for removing carbon particulate.

EXAMPLE 6

An exhaust gas purifying device was manufactured by disposing a catalystdownstream of the plasma generation device of Example 1. The NO_(x)purification performance of the exhaust gas purifying device wasevaluated. As the catalyst, a catalyst powder prepared by impregnatingcommercially-available γ-Al₂O₃ with 5 mass % of Pt was supported on acordierite ceramic honeycomb. The honeycomb catalyst was in the shape ofa cylinder having a diameter of 1 in (about 2.54 cm) and a length of 60mm. The number of cells was 400, and the thickness (rib thickness) ofthe partition walls partitioning the cells was 4 mil (about 0.1 mm). Theplasma generation conditions and the gas conditions were the same asthose of Example 1 (7 kV).

As a result, the NO_(x) concentration of the mixed gas having an NOconcentration of 200 ppm was reduced to 80 ppm after the mixed gas hadpassed the plasma generation device and the catalyst.

COMPARATIVE EXAMPLE 2

An exhaust gas purifying device was manufactured by disposing a catalystsimilar to that used in Example 6 downstream of the plasma generationdevice of Comparative Example 1. The NO_(x) purification performance ofthe exhaust gas purifying device was evaluated. The plasma generationconditions and the gas conditions were the same as those of ComparativeExample 1.

As a result, the NO_(x) concentration of the mixed gas having an NOconcentration of 200 ppm was reduced little to 110 ppm after the mixedgas had passed the plasma generation device and the catalyst.

INDUSTRIAL APPLICABILITY

A plasma generating electrode and a plasma generation device of thepresent invention can generate uniform and stable plasma at low powerconsumption. Since an exhaust gas purifying device of the presentinvention includes the aforementioned plasma generation device and acatalyst, the exhaust gas purifying device can suitably be used as apurifying device which purifies, for example, exhaust gas dischargedfrom an automotive engine or the like.

1. A plasma generating electrode comprising at least a pair ofelectrodes disposed opposite to each other and generating plasma uponapplication of voltage between the pair of electrodes, at least one ofthe pair of electrodes including a ceramic plate having two majorsurfaces as a dielectric and a conductive film disposed inside theceramic plate sandwiched between the two major surfaces and having aplurality of through-holes formed through the conductive film in itsthickness direction, the through-holes having a cross-sectional shapeincluding an arc shape along a plane perpendicular to the thicknessdirection, wherein a cross-sectional area of the through-holes have adiameter of 1 to 10mm.
 2. The plasma generating electrode according toclaim 1, wherein the through-holes have a circular cross-sectional shapealong the plane perpendicular to the thickness direction.
 3. The plasmagenerating electrode according to claim 1, wherein the through-holes areregularly arranged in the conductive film.
 4. The plasma generatingelectrode according to claim 1, wherein the conductive film is disposedinside the ceramic plate by screen printing, calender rolling, spraying,chemical vapor deposition, or physical vapor deposition.
 5. The plasmagenerating electrode according to claim 1, wherein a center-to-centerdistance between the adjacent through-holes is 1.5 to 20 mm.
 6. Theplasma generating electrode according to claim 1, wherein the conductivefilm includes at least one metal selected from the group consisting oftungsten, molybdenum, manganese, chromium, titanium, zirconium, nickel,iron, silver, copper, platinum, and palladium as a major component.
 7. Aplasma generation device comprising the plasma generating electrodeaccording to claim
 1. 8. An exhaust gas purifying device comprising theplasma generation device according to claim 7 and a catalyst, the plasmageneration device and the catalyst being disposed inside an exhaustsystem of an internal combustion engine.